Gilbert Kennedy, 3rd Earl of Cassilis was Scottish landowner, soldier, politician, and judge. He served as Treasurer of Scotland. The son of Gilbert Kennedy, 2nd Earl of Cassilis, he succeeded to the titles of 5th Lord Kennedy and 3rd Earl of Cassillis in August 1527. On 6 February 1540/41 he had a charter of the Fief of Cassilis. As a young man, Kennedy studied at the University of St. Andrews and in Paris under the Scottish humanistGeorge Buchanan for five years. In November, 1542, Kennedy, in his late twenties at the time, was taken prisoner at the Battle of Solway Moss, and after a short stint in the Tower of London, was placed under the care ofThomas Cranmer, Archbishop of Canterbury. Scottish historian Gilbert Burnet believed that it was this relationship with Cranmer that led Kennedy toward Protestantism, as certainly he was one of the first of the Scottish nobility to adopt Reformed views. However, since his time with Cranmer lasted only one month, it is likely that his earlier contact with Buchanan had been a stronger influence in this regard. Sir Thomas Wharton, Warden of the English West March, wrote to the Privy Council of King Henry VIII, on the tenth of December 1542, regarding ransoms for prisoners taken at the battle, including the Earl of Cassilis, who was taken by Batill Routlege. Credit was also given to John Musgrave who "claimeth a part for the loan of his horse to the said Routlege." Cassilis held the office of Treasurer of Scotland in 1554, of Extraordinary Lord of Session between 1546 and 1558. He fought in the Battle of Pinkie Cleugh on 10 September 1547. Cassilis borrowed money from Timothy Cagnioli, an Italian financier, for his expenses as a diplomat in France in 1558. The sum of £6,720 Scots was not repaid in 1586, and Cagnioli claimed it from his grandson, John Kennedy, 5th Earl of Cassilis. In 1558 he was present, as one of the eight Commissioners appointed by the Scottish Parliament, at the marriage of Mary, Queen of Scots, to the Dauphin of France, to whom the Scottish deputies unanimously refused the Crown matrimonial. The Court of France appeared deeply mortified by this disappointment, and the Earl of Cassilis, with two others of the Commissioners, dying in one night, on 28 November 1558, at Dieppe, a report was raised that they had been poisoned, which was further countenanced by the death of a fourth Commissioner, Lord Fleming, at Paris, on 16 December 1558. However, according to the Dictionary of National Biography, this report was untrue, as the actual dates of death of these persons were not the same, Cassilis dying in November. Henry II of France had appointed Gilbert as Gentleman in Ordinary of his Chamber on 4 May 1558, and this honour was transferred to his heir, Gilbert, 4th Earl of Cassilis on 10 February 1559.
